In recent years, MLS cylinder head gaskets for internal combustion engines have become a preferred choice with at least two gasket layers formed from steel. In a typical MLS gasket, the layers are formed with a plurality of openings. More specifically, typical gaskets include cylinder bore openings, coolant openings, bolt holes and oil holes. Typically, the gasket includes full beads around the cylinder bore openings and half beads that surround the non-cylinder bore openings and the outer periphery of the gasket. The bolt holes, which are generally located around the periphery of the gasket, cooperate with bolts to secure the gasket between the cylinder head and engine block. The cylinder bore openings, coolant holes and oil holes are sealed by surface pressures formed by the clamping action of the bolts to deform the half or full beads, thereby creating a sealing pressure between the portions of the cylinder head and engine block that are in contact with the gasket.
Generally, when the bolts are tightened, the surface pressure between portions of the cylinder head and engine block that are in contact with the gasket varies somewhat throughout the sealing joint. Since the seal around the cylinder bore openings is generally a primary concern in head gasket manufacture, other sealing portions, usually half beads, of the gasket must accommodate this priority of design. During engine operation, the high pressures and temperatures, and the operating conditions at the head gasket location cause movement between the cylinder head and the cylinder block. This movement causes variation in sealing pressures and may cause the full beads or half beads to compress until bead portions contact a surface that inhibits further compression. This variation in sealing pressure is especially prevalent in the full beads surrounding the cylinder bore openings.
In practice, no cylinder head and cylinder block mating surfaces are perfectly planar or perfectly parallel. Gaps between the cylinder head and cylinder block exist if an engine were to be assembled without a gasket interposed therebetween. With a gasket installed between the cylinder head and cylinder block, areas of localized stress are created when fasteners (not shown) such as a bolt are positioned through bolt holes, and then tightened to secure the cylinder head and cylinder block. These gaps result in unequal sealing stresses around the bolt holes, the cylinder bore openings, and other portions of the gasket. Furthermore, the gasket will be subject to high loads in compression during engine operation. This high compression loading generates higher stresses that may result in an undesirable reduction in recovery capability and/or characteristics of the gasket, such as cracking of bead portions or plastic deformation of bead portions.
FIG. 1 illustrates a portion of a MLS gasket 20. Gasket 20 has a body, or base layer 22, a top layer 24, and a bottom layer 26. A cylinder aperture 30 is formed in the layers 22, 24, 26. Both the top layer 24 and the bottom layer 26 have a bead 32 formed therein that circumscribes the cylinder aperture 30. Base layer 22 further include an embossed edge stopper 40 formed thereon. The beads 32 and the embossed edge stopper 40 circumscribe the cylinder aperture 30 to exert a sealing pressure around the cylinder aperture 30 and between a cylinder head (not shown) and an engine block (not shown).
In operation, the gasket 20 introduces stresses into an engine, in part, due to the edge stopper 40 impacting the top layer 24 adjacent bead 32 during relative vertical motion between the cylinder head and engine block. Specifically, the location of the edge stopper adjacent the cylinder aperture 30 results in distortion of both the cylinder head and engine block, especially when aluminum is utilized. Undesirable distortion of the cylinder bore may effect engine performance and ring sealing capability. Significant stresses are encountered where the valve seats formed within an aluminum cylinder head are distorted. Since steel valve seats are often inserted within aluminum cylinder heads, the coupling between the valve seats and the cylinder head may be undesirably degraded due to the additional stresses introduced by a stopper such as the edge stopper 40. Also, embossments such as the edge stopper 40 may be damaged, or flattened in portions during severe or normal operations, which may lead to premature bead failure since the embossments may no longer protect the beads from undesirable full compression.
Additionally, all layers of a MLS gasket are typically coated stainless steel. The coatings on the gasket layers are used to improve gasket performance and are typically more difficult and costly to apply to a stainless steel as compared to a carbon steel.
Evaluations have been performed on gaskets that have experienced an undesirable reduction in recovery capability and/or characteristics at portions, such as the sealing beads surrounding cylinder apertures in gaskets that are interposed between a cylinder head and a cylinder block of an internal combustion engine (not shown). In many gaskets evaluated, the undesirable reduction in recovery capability and/or characteristics of bead portions were greater with edge stoppers. Accordingly, there is a need to provide a sealing mechanism that eliminates edge stoppers and may permit beads to be formed of a material with a greater stiffness than is permitted with an optimized edge stopper type gasket. A favorable gasket may also reduce the costs and difficulty associated with coating the surface area of all gasket layers.